How to make a (mesh) light only cast a shadow?
1: Negative lighting

Add a lamp and an idetical one but with negative strength and shadow casting disabled:

Positive Lamp



Negative Lamp



Without the Lamps



With the Lamps



Additional Notes

Because mesh light shadow casting can not be disabled, this works only for other light sources. Requires more samples than without the shadows. These are subtractive shadows, meaning the value of the shadow is subtracted from the color. Multiplicative shadows would look better, but those require spearate render layers and compositing.

2: Compositing

On the render layers tab there is an option for quickly replacing all materials with a single one, which can help with creating a shadow map render for the light by using a simple white material. You could exposure and gamma adjust the result of the render layer and multiply it ower the original render.

Additional Notes

This is a little longer setup, but with material overriding and putting the main lights and the shadow light to separate layers it can be easier. It might be good to disable bounces for the shadow render. This method however, instead of casting only shadows also darkens faces facing away from the light.